1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to a system and method for performing a ranging process in a communication system.
2. Description of the Related Art
Hereinafter, a description will be made based on an IEEE (Institute of Electrical and Electronics Engineers) 802.16 Orthogonal Frequency Division Multiple Access (OFDMA) system.
FIG. 1 is a schematic block diagram illustrating the structure of a general Broadband Wireless Access (BWA) communication system. Referring to FIG. 1, a Subscriber Station (SS) 10 generally has mobility and is connected to a Backbone Network (BN) 30 through a Base Station (BS) 20. The SS 10 allows connection between the BS 20 and a subscriber. The BS 20 provides control, management, and connectivity for the SS 10. The BN 30 is connected to an Authentication and Service Authorization (ASA) server 40 for authentication and service authentication of the SS 10.
The SS 10 needs to successfully complete a network entry process with the BS 20 to communicate on the BWA communication system. The network entry process is divided into downlink channel synchronization, initial ranging, capabilities negotiation, authentication message exchanges, registration, and Internet Protocol (IP) connectivity stages. Upon completion of the network entry process, the SS 10 creates one or more service flows to send data to the BS 20. The initial ranging in the network entry process will be described with reference to FIG. 2.
FIG. 2 is a schematic flow diagram for illustrating the initial ranging in the BWA communication system. The SS 10 begins the initial ranging by sending an initial ranging code to the BS 20 in a contention-based manner in step 101. Such code ranging allows adjustment of UpLink (UL) timing and power offset of the SS 10. The BS 20 marks in a Ranging Response (RNG_RSP) message code ranging information and a power offset adjustment value transmitted from the SS 10 and broadcasts the RNG_RSP message in step 103. Upon receipt of the RNG_RSP message, the SS 10 checks for the code ranging information and adjusts the power offset. If a status field of the RNG_RSP message indicates ‘continue’, the SS 10 re-attempts the code ranging in step 105.
If the BS 20 determines that timing and power adjustment are completed through the code ranging attempted by the SS 10, it marks the status field of the RNG_RSP message as ‘success’ and sends the RNG_RSP message to the SS 10 in step 107. The BS 20 allocates a UL bandwidth in which the SS 10 can transmit a Ranging Request (RNG_REQ) message through UL-MAP having a Code Division Multiple Access (CDMA)_Allocation_Information Element (IE). The CDMA_Allocation_IE includes the code ranging information transmitted from the SS 10 and the amount of UL resource allocated to the SS 10. If the status field of the RNG_RSP message indicates ‘success’, the SS 10 terminates the code ranging and attempts message ranging.
The SS 10 transmits the RNG_REQ message in the allocated UL bandwidth in step 111. Once the SS 10 receives the RNG_RSP message from the BS 20 in step 113, the ranging is completed. The SS 10 is allocated a basic Connection IDentifier (CID) and a primary CID through the ranging. These CIDs are uniquely allocated to the SS 10 from the BS 20 and are used to identify the SS 10 during an access to the BS 20. Information such as MAC address information of the SS 10 is transmitted through the RNG_REQ message. The BS 20 responds to the RNG_REQ message with the RNG_RSP message based on such information. In step 115, the SS 10 sends an SS Basic Capability Request (SBC_REQ) message through the basic CID included in the RNG_RSP message to perform capabilities negotiation.
The RNG_REQ message may include information as a Type/Length/Value (TLV) form. Since information included in a single RNG_REQ message may vary with implementation or situations, the size of the RNG_REQ message may also vary.
However, since the SS 10 is not allocated a unique CID when transmitting the RNG_REQ message, i.e., before receiving the RNG_RSP message, it cannot request the BS 20 to allocate (UL) bandwidth. If the BS 20 fails to receive the RNG_REQ message from the SS 10 in the allocated UL bandwidth, it cannot determine whether such a failure is due to a packet loss or that the SS 10 did not attempt to transmit the RNG_REQ message. Thus, the SS 10 can transmit the RNG_REQ message only after performing code raging again and being allocated the CDMA_Allocation_IE from the BS 20. In this case, since it is not guaranteed that the BS 20 will allocate a sufficient amount of resource to the SS 10 through the CDMA_Allocation_IE, the RNG_REQ message transmission problem still remains.